JP4875767B2 - Integrator, resonator and oversampling A / D converter - Google Patents

Description

  The present invention relates to an integrator, and more particularly to an integrator suitable for a continuous-time ΔΣ modulator.

The oversampling A / D converter is widely used for the front end of communication equipment, audio signal conversion, and the like, and is an essential circuit technology for current communication, video, and audio signal processing circuits. One of the oversampling A / D converters is a continuous-time delta-sigma A / D converter (CTDS-ADC) having a continuous-time filter (for example, non-patent literature). 1 and 2).

In general CTDS-ADC, an input signal is quantized by a quantizer through n cascaded integrators (continuous time filters). The digital output of the quantizer is converted into an analog current signal by n D / A converters and then fed back to each of n integrators. In the CTDS-ADC, since the analog circuit portion does not include a switch, the voltage can be reduced. In addition, a pre-filter that is normally required when a sampling filter is used is not necessary in the CTDS-ADC. From these points, the CTDS-ADC is suitable for application to a communication system, and application development research has been actively conducted in recent years.
Richard Schreier and Bo Bang, "Delta-Sigma Modulators Employing Continuous-Time Circuitry", IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS--I: FUNDAMENTAL THEORY AND APPLICATIONS, VOL. 43, NO. 4, APRIL 1996 Xuefeng Chen et al., "A 18mW CT ΔΣ Modulator with 25MHz Bandwidth for Next Generation Wireless Applications", IEEE 2007 Custom Intergrated Circuits Conference, 2007

  In order to improve resolution and SN performance in CTDS-ADC, it is necessary to increase the filter order for removing quantization noise, and operational amplifiers for that order are required. That is, in order to improve the performance of the CTDS-ADC, a large number of operational amplifiers must be used. However, an increase in the number of operational amplifiers leads to an increase in circuit scale and power consumption, which contributes to a bottleneck for improving the performance of a system LSI applied to portable communication devices and the like.

In view of the above problems, an object of the present invention is to provide an integrator that exhibits high-order integration characteristics with one operational amplifier. It is another object of the present invention to provide a resonator using such an integrator and a continuous-time oversampling A / D converter.

  In order to solve the above problems, the present invention has taken the following measures. An integrator having an operational amplifier, a first filter connected to the inverting input terminal of the operational amplifier, and a second filter connected between the inverting input terminal and the output terminal of the operational amplifier; To do. Here, the first filter includes n resistance elements connected in series, and n-1 capacitance elements having one end connected to each connection point of the resistance elements and the other end grounded. The second filter includes n capacitive elements connected in series, and n-1 resistive elements having one end connected to each connection point of the capacitive elements and the other end grounded. However, n is an integer of 2 or more.

According to this, by making a predetermined relationship hold for each element value of the resistive element and the capacitive element in the first filter and the resistive element and the capacitive element in the second filter, the transfer function of the integrator is In the denominator and numerator, the terms from s (where s is a Laplace operator) to s ^n-1 are canceled to become only 1 / s ⁿ terms. That is, an n-order integrator can be configured with one operational amplifier.

  The integrator preferably further includes a third filter having at least one of a resistance element and a capacitance element connected in parallel to the first filter. According to this, the third filter acts as a feedforward path between the input of the integrator and the inverting input terminal of the operational amplifier. As a result, zero-order, first-order, and second-order integral components can be generated in the output of the integrator.

  If at least one Gm element or resistance element connected between at least one connection point of the resistance element in the first filter and the output terminal of the operational amplifier is added to the integrator, a resonator is formed. Can do.

  An oversampling A / D converter according to the present invention includes the integrator, a quantizer that quantizes the output of the integrator, and a digital output of the quantizer that converts the digital output into a current signal. N−1 D / A converters that feed back to each connection point of the resistive element in the filter and the digital output of the quantizer are converted into current signals and fed back to each connection point of the capacitive element in the second filter. n-1 D / A converters. Alternatively, an oversampling A / D converter according to the present invention includes the integrator described above, a quantizer that quantizes the output of the integrator, and a digital output of the quantizer that is converted into a current signal. N-1 D / A converters that feed back to each connection point of the resistive elements in the filter, and a D / A converter that converts the digital output of the quantizer into a current signal and feeds it back to the inverting input terminal of the operational amplifier And. Alternatively, an oversampling A / D converter according to the present invention includes an integrator including the third filter, a quantizer that quantizes the output of the integrator, and a digital output of the quantizer as a current signal. And a D / A converter that feeds back to the input side of the integrator. These oversampling A / D converters exhibit higher-order filtering characteristics than the number of operational amplifiers included therein.

  When at least one Gm element or resistance element connected between at least one of the connection points of the resistance element in the first filter and the output terminal of the operational amplifier is added to the oversampling A / D converter. The filtering characteristic having a zero point in the transmission characteristic of the quantization noise is exhibited.

  According to the present invention, a small and low power consumption n-order integrator and n-order resonator can be obtained. Furthermore, it is possible to reduce the size and power consumption of the oversampling A / D converter with high resolution and high S / N ratio.

FIG. 1 is a configuration diagram of an integrator according to the first embodiment. FIG. 2 is a configuration diagram of an integrator according to the second embodiment. FIG. 3 is a configuration diagram of an integrator according to the third embodiment. FIG. 4 is a configuration diagram of an oversampling A / D converter according to the fourth embodiment. FIG. 5 is a configuration diagram of an oversampling A / D converter according to the fifth embodiment. FIG. 6 is a configuration diagram of an oversampling A / D converter according to the sixth embodiment. FIG. 7 is a graph showing a simulation result of the transient response of the resonator according to the present invention shown in FIG. FIG. 8 is a configuration diagram of an oversampling A / D converter according to the seventh embodiment. FIG. 9 is a graph showing a simulation result of the transient response of the resonator according to the present invention shown in FIG. FIG. 10 is a configuration diagram of an oversampling A / D converter according to the eighth embodiment. FIG. 11 is a configuration diagram of an oversampling A / D converter according to the ninth embodiment.

10 integrator 10A, 10A 'resonator 11 operational amplifier 12 filter (first filter)
121 Resistance element 122 Capacitance element 13 Filter (second filter)
131 Capacitance element 132 Resistance element 14 Filter (third filter)
141 Resistance element 142 Capacitance element 15 Gm element 16 Resistance element 20 Quantizer 30 D / A converter 40 D / A converter

  The best mode for carrying out the present invention will be described below with reference to the drawings.

(First embodiment)
FIG. 1 shows a configuration of an integrator according to the first embodiment. The integrator 10 includes an operational amplifier 11, a filter 12 connected to the inverting input terminal of the operational amplifier 11, and a filter 13 connected between the inverting input terminal and the output terminal of the operational amplifier 11. The filter 12 is a secondary low-pass filter including two resistance elements 121 connected in series, and a capacitance element 122 having one end connected to a connection point of these resistance elements and the other end grounded. The filter 13 is a secondary high-pass filter including two capacitive elements 131 connected in series, and a resistive element 132 having one end connected to a connection point of these capacitive elements and the other end grounded.

In the integrator 10, the input voltage is V _in , the output voltage is V _out , the resistance value of the resistance element 121 is R ₁ , the capacitance value of the capacitance element 122 is C ₁ , the resistance value of the resistance element 132 is R ₂ , and the capacitance element 131. , C ₂ , the voltage at the connection point between the resistance element 121 and the capacitance element 122 as V ₁ , and the voltage at the connection point between the capacitance element 131 and the resistance element 132 as V ₂ , the following nodal equation is derived. Is done. Here, s is a Laplace operator.

  When this nodal equation is solved, the transfer function of the integrator 10 is derived as follows.

Here, when C ₁ R ₁ = 4C ₂ R ₂ holds, the following transfer function is derived.

That is, by appropriately setting the element values of the resistance elements 121 and 132 and the capacitance elements 122 and 131, a transfer function consisting only of the 1 / s ² term can be obtained. Thus, according to the present embodiment, a single-stage operational amplifier 11 can constitute a secondary integrator. Note that a fourth-order or higher order integrator can be configured by multiplexing the integrators 10 according to the present embodiment. For example, by multiplexing two integrators 10 according to this embodiment, a four-order integrator can be configured with two operational amplifiers 11.

(Second Embodiment)
FIG. 2 shows a configuration of an integrator according to the second embodiment. The integrator 10 includes filters 12 and 13 having a configuration different from that of the integrator 10 of FIG. That is, the filter 12 includes three or more resistance elements 121 connected in series, one end connected to each connection point of these resistance elements, the other end grounded, and one capacitive element 122 less than the resistance element 121. An n-order low-pass filter provided. The filter 13 includes three or more capacitive elements 131 connected in series, one end connected to each connection point of these capacitive elements, the other end grounded, and one less resistive element 132 than the capacitive element 131. It is an nth order high pass filter.

  When the number of the resistive elements 121 and the capacitive elements 131 is n, the transfer function of the integrator 10 is generally expressed by the following equation. However, α, β, γ, τ, and κ are constants determined by the element values of the resistance elements 121 and 132 and the capacitance elements 122 and 131.

Here, by establishing a predetermined relationship between the element values of the resistance elements 121 and 132 and the capacitance elements 122 and 131, the terms from s to s ^n-1 are canceled in the denominator and numerator of the transfer function. Thus, a transfer function consisting only of 1 / s ⁿ terms is obtained. Thus, according to the present embodiment, an n-order integrator can be configured with one operational amplifier.

In each of the above embodiments, the two resistance elements 121 in the filter 12 may have different resistance values. The two capacitive elements 131 in the filter 13 may also have different capacitance values . In any case, the second-order or higher-order integrator 10 can be configured by satisfying a predetermined relationship between the element values of the resistance elements 121 and 132 and the capacitance elements 122 and 131.

(Third embodiment)
FIG. 3 shows a configuration of an integrator according to the third embodiment. The integrator 10 is obtained by adding a filter 14 to the integrator 10 of FIG. 1 or FIG. The filter 14 includes a resistance element 141 and a capacitance element 142 connected in parallel to the filter 12. The filter 14 acts as a feedforward path between the input of the integrator 10 and the inverting input terminal of the operational amplifier 11. As a result, in addition to the nth-order integral component, zeroth-order, first-order and second-order integral components can be generated in the output of the integrator 10. The integral component of each order can be adjusted by appropriately setting the element values of the resistor element 141 and the capacitor element 142.

  Note that one of the resistor element 141 and the capacitor element 142 may be omitted. For example, when the filter 14 is composed of only the resistance element 141, a first-order integral component can be generated in the output of the integrator 10 in addition to the n-order integral component. When the filter 14 is composed of only the capacitive element 142, zero-order and first-order integral components can be generated at the output of the integrator 10.

(Fourth embodiment)
FIG. 4 shows a configuration of a CTDS- ADC according to the fourth embodiment. The CTDS-ADC according to the present embodiment includes an integrator 10 in FIG. 1, a quantizer 20 that quantizes the output of the integrator 10, a digital output of the quantizer 20 into a current signal, and a resistance element in the filter 12. A D / A converter 30 that feeds back to a connection point 121 and a D / A converter 40 that converts the digital output of the quantizer 20 into a current signal and feeds back to the connection point of the capacitive element 131 in the filter 13 are provided. . Here, when the output current of the D / A converter 30 is I ₁ and the output current of the D / A converter 40 is I ₂ , the following nodal equation is derived.

This relationship of solving the nodal equation and the output voltage V _out and the current I ₁ of the integrator 10 is derived as follows.

Here, when C ₁ R ₁ = 4C ₂ R ₂ holds, the following relational expression is derived.

Also, detailed calculation process is omitted, the relationship between the output voltage V _out and the current I ₂ of the integrator 10 is derived as follows.

  That is, by appropriately setting the element values of the resistance elements 121 and 132 and the capacitance elements 122 and 131, the output of the D / A converter 30 is fed back to the connection point of the resistance element 121 in the filter 12 to perform the second-order integration operation. It can be performed. Further, the output of the D / A converter 40 can be fed back to the connection point of the capacitive element 131 in the filter 13 to perform the first order integration operation. Thus, according to the present embodiment, an oversampling A / D converter having secondary filtering characteristics (quantization noise elimination characteristics) can be configured with one operational amplifier 11. An oversampling A / D converter that exhibits fourth-order or higher filtering characteristics can be configured by multiplexing the integrator 10. For example, by multiplexing two integrators 10, an oversampling A / D converter that exhibits fourth-order filtering characteristics can be configured with two operational amplifiers 11.

(Fifth embodiment)
FIG. 5 shows a configuration of a CTDS- ADC according to the fifth embodiment. The CTDS-ADC according to the present embodiment includes an integrator 10 in FIG. 2, a quantizer 20 that quantizes the output of the integrator 10, a digital output of the quantizer 20 into a current signal, and a resistive element in the filter 12. A plurality of D / A converters 30 that feed back to each connection point 121 and a plurality of D / A conversions that convert the digital output of the quantizer 20 into a current signal and feed back to each connection point of the capacitive element 131 in the filter 13. A container 40 is provided. As described above, the integrator 10 exhibits the n-th order integration characteristic by satisfying a predetermined relationship between the element values of the resistance elements 121 and 132 and the capacitance elements 122 and 131. Thus, according to the present embodiment, an oversampling A / D converter that exhibits n-th order filtering characteristics can be configured with one operational amplifier 11.

(Sixth embodiment)
FIG. 6 shows a configuration of a CTDS-ADC according to the sixth embodiment. The CTDS-ADC according to the present embodiment includes a general primary integrator 10 ′ and secondary resonators 10A and 10A ′, and exhibits a fifth-order filtering characteristic as a whole. In FIG. 6, symbols shown beside each element represent each element value. The resonators 10A and 10A ′ are obtained by adding a Gm element (transconductor element) 15 to the integrator 10 of FIG. The Gm element 15 feeds back the output of the operational amplifier 11 to the connection point of the resistance element in the filter 12.

  FIG. 7 shows a simulation result of the transient response of the resonator 10A when a 5 MHz sine wave near the resonance frequency is input to the resonator 10A shown in FIG. However, the gain of the operational amplifier 11 is 70 dB, and GBW is 200 MHz. From this simulation result, it can be seen that the resonator 10A does not oscillate.

(Seventh embodiment)
FIG. 8 shows a configuration of a CTDS-ADC according to the seventh embodiment. The CTDS-ADC according to this embodiment is obtained by replacing the Gm element 15 in the resonators 10A and 10A ′ with the resistance element 16 in the CTDS-ADC of FIG.

  FIG. 9 shows a simulation result of the transient response of the resonator 10A when a 5 MHz sine wave near the resonance frequency is input to the resonator 10A shown in FIG. However, the gain of the operational amplifier 11 is 70 dB, and GBW is 200 MHz. From this simulation result, it can be seen that the resonator 10A does not oscillate. Furthermore, when compared with the graph of FIG. 7, it can be seen that the resonator 10A according to the present embodiment has better transient response characteristics than the resonator 10A according to the sixth embodiment.

(Eighth embodiment)
FIG. 10 shows a configuration of a CTDS-ADC according to the eighth embodiment. The CTDS-ADC according to the present embodiment includes a general first-order integrator 10 ′ and a third-order resonator 10A, and exhibits a fourth-order filtering characteristic as a whole.

  The resonator 10A is obtained by adding three Gm elements 15 to the integrator 10 of FIG. Each Gm element 15 feeds back the output of the operational amplifier 11 to each connection point of the resistance element in the filter 12. Similarly to the CTDS-ADC in FIG. 5, the outputs of the three D / A converters 30 are fed back to the connection points of the resistive elements in the filter 12. On the other hand, unlike the CTDS-ADC of FIG. 5, the output of the D / A converter 40 is fed back to the inverting input terminal of the operational amplifier 11 instead of the connection points of the capacitive elements in the filter 13. Thus, according to this embodiment, the total number of D / A converters can be relatively small, and the circuit scale and power consumption can be reduced.

  The Gm element 15 may be replaced with a resistance element. Further, it is not necessary to connect the Gm element 15 or the resistance element to all connection points of the resistance element in the filter 12. That is, a resonator can be configured by feeding back the output of the operational amplifier 11 to at least one connection point of the resistive element in the filter 12 via the Gm element or the resistive element.

(Ninth embodiment)
FIG. 11 shows the configuration of a CTDS-ADC according to the ninth embodiment. The CTDS-ADC according to the present embodiment includes a general first-order integrator 10 ′ and a second-order resonator 10A, and exhibits third-order filtering characteristics as a whole. The resonator 10 </ b> A is obtained by adding a resistance element 16 that feeds back the output of the operational amplifier 11 to the connection point of the resistance element in the filter 12 to the integrator 10 of FIG. 3.

  In the resonator 10A, the filter 14 acts as a feedforward path between the input of the resonator 10A and the inverting input terminal of the operational amplifier 11. Therefore, the phase compensation of the CTDS-ADC can be performed without feeding back the output of the quantizer 20 to the connection point of the resistive element in the filter 12. Therefore, according to this embodiment, the number of D / A converters can be further reduced, and the circuit scale and power consumption can be further reduced.

  The resistance element 16 may be replaced with a Gm element. Further, the resistance element 16 may be omitted. In this case, the CTDS-ADC exhibits a filtering characteristic that does not have a zero point in the transmission characteristic of the quantization noise.

  In each of the above embodiments, each resistive element may be configured with a switched capacitor circuit. Thereby, the integrator 10 and the resonators 10A and 10A 'can be made into discrete filters. Since the discrete filter can determine the transfer function of the circuit by the capacitance ratio, the filtering accuracy can be improved.

  The integrator, the resonator, and the oversampling A / D converter according to the present invention exhibit high-order integration characteristics and filtering characteristics with a relatively small scale and low power consumption, and thus are useful for portable communication devices and the like.

Claims (8)

A signal input terminal;
A signal output end;
An operational amplifier having an output connected to the signal output ;
A first filter connected between the signal input terminal and an inverting input terminal of the operational amplifier;
And a second filter connected between the inverting input terminal and the output terminal of said operational amplifier,
In the first filter, n is an integer of 2 or more,
N resistance elements connected in series between the signal input terminal and the inverting input terminal of the operational amplifier ;
And n-1 capacitance elements inserted and connected between the n-1 connection points for individually connecting the n resistance elements and a ground node ,
The second filter is:
N capacitive elements connected in series between the inverting input terminal and the output terminal of the operational amplifier ;
An integrator having n-1 connection points for individually connecting the n capacitive elements and n-1 resistance elements inserted and connected between the ground nodes . . The integrator of claim 1, wherein
An integrator comprising a third filter having at least one of a resistance element and a capacitance element connected in parallel to the first filter. The integrator of any one of claims 1 and 2,
The integrator is characterized in that each of the resistance elements is a switched capacitor circuit. An integrator according to any one of claims 1 to 3;
And at least one Gm element or a resistive element connected between the output end of at least one of the operational amplifiers of the (n-1) of the connection point in the first filter,
A resonator, wherein a signal is input to the signal input terminal of the integrator and a signal is output from the signal output terminal of the integrator . An integrator according to claim 1;
A quantizer for quantizing the output of the integrator;
And (n-1) D / A converter is fed back to the (n-1) each connection point in the first filter digital output is converted to a current signal of the quantizer,
And a (n-1) D / A converter for feeding back by converting the digital output of the quantizer to a current signal to the (n-1) each connection point in the second filter,
An oversampling A / D converter , wherein an analog signal is input to the signal input terminal of the integrator, and the digital output of the quantizer is used as an output signal . An integrator according to claim 1;
A quantizer for quantizing the output of the integrator;
And (n-1) D / A converter is fed back to the (n-1) each connection point in the first filter digital output is converted to a current signal of the quantizer,
And a D / A converter is fed back to the inverting input terminal of the quantizer of the digital output is converted into a current signal to the operational amplifier,
An oversampling A / D converter , wherein an analog signal is input to the signal input terminal of the integrator, and the digital output of the quantizer is used as an output signal . An integrator according to claim 2;
A quantizer for quantizing the output of the integrator;
A D / A converter that converts the digital output of the quantizer into a current signal and feeds it back to the input side of the integrator ;
An oversampling A / D converter , wherein an analog signal is input to the signal input terminal of the integrator, and the digital output of the quantizer is used as an output signal . The oversampling A / D converter according to any one of claims 5 to 7,
Over characterized in that it comprises at least one Gm element or a resistive element connected between the output end of at least one of the operational amplifiers of the (n-1) of the connection point in said first filter Sampling A / D converter.

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